External Influences on Hurricane Intensity. Part III: Potential Vorticity Structure

Abstract

The interaction of Hurricane Elena (1985) with a baroclinic wave was reexamined using both potential vorticity (PV) and a formulation for Eliassen-Palm fluxes in cylindrical coordinates. The hurricane began to deepen rapidly as a narrow upper-level positive PV anomaly became nearly superposed over the low-level center. The intensification appeared to represent an evaporation-wind feedback activated by constructive interference of the PV anomalies. Enhanced convection associated with this process eroded the upper PV anomaly and prevented it from crossing the hurricane and reversing the intensification. The Eliassen?Palm flux divergence showed that maximum eddy activity remained in the upper troposphere prior to the reintensification. This activity was produced in part because the action of the outflow anticyclone of the hurricane contributed to synoptic-scale wave breaking. The upper-level PV anomaly that approached the center was much narrower in extent than the original synoptic-scale trough. The deep layer of vertical wind shear that would have prevented intensification of the hurricane was avoided. It is concluded that the interaction of a tropical cyclone and a synoptic-scale trough cannot be viewed simply as the bringing together of positive PV anomalies. Rather, the outflow anticyclone, constantly reinforced by the large source of low PV air from the storm core, interacts with and resists shearing by the trough. Whether the hurricane intensifies during such interactions depends to a large extent upon the relative strengths of these positive and negative PV anomalies. Such outflow-layer interactions represent a fruitful area for further research into tropical cyclone intensity change.